NFC Antenna Theory

I've been thinking about NFC antennas for a while now, working in the field of Radio Communications antennas are something that I deal with a lot.
Antenna length is a function of wave length, that is the distance between peak and peak of a sinusoidal wave in a radiated frequency.
This looks like the curved up and down wave that you tend to see on oscilloscope pictures.
With a normal radio system, lets say 27MHz CB since that is fairly prevalent around the world, the wavelength between peak and peak is around 36 feet. To make things a little easier on everyone we deal in 5/8 wave, half-wave and quarter-wave antennas which makes the electrical length of the antenna a lot easier to cope with and has us using much more compact designs.
So if any of you remember the bad old days of CB, it wasn't that odd to see cars getting about with 9 foot tall quarter-wave stainless steel whip antennas - this being the perfect electrical quarter wave for those frequencies.
There are all sorts of tricks out there to reduce the length but keep it electrically the same, but that's a lot of antenna theory for a post on a Monday morning. Suffice to say that at 27MHz you need a big antenna.

Now we have a frame of reference for things I'll drop the next thing on you - With RF, as your frequency goes down your wavelength goes up.
NFC functions at around 13.56MHz, and that leaves us with a wavelength of around 72 feet.
If we cut that down to a quarter wave, we still need an antenna that is 18 feet long if we want an efficiently functioning radiating system.

I don't know how big your hands are, but a ring carrying that around is going to make me uncomfortable!
Luckily, science.
We've gotten to the point where we realise that there is no way we're going to fit 18 feet worth of antenna into a smartphone or a ring ever and so we conveniently ignore antenna theory and concentrate instead on coupling loop inductors.

What this means is that the NFC antennas are not really antennas at all, merely closely matched loops of wire in each device, with the active device like the smartphone inducing a current in the NFC tag which is inside the magnetic field of the reader.

At it's most absolute basic form, the more loops you have and the more closely matched they are, the better the transmission between devices will be. In your paywave/paypass credit card or NFC access card those loops are around the outside edge of the card and they're positioned similarly in most credit card style readers. As this is a close match the devices work with minimal fuss, positioning of the card on the reader doesn't have to be perfect but still has to be close, because as antennas their efficiency is close to zero.

When we get smaller NFC devices like keyring tags, the loops are somewhat smaller in the tag than in the reader, this means that we're now functioning with somewhat less efficiency than before and while they will still function we need to position them on the reader fairly carefully in order to have them activated. Where the credit card could be read from a small distance, the smaller tag has to be right on the reader before it will function.

And so now we come to the NFC Ring, which is a tiny NFC tag and needs to be placed just in that perfect sweet spot to be affected by the electromagnetic field of the reader device. This is because the ring's loops are so much smaller than the average reader device's loops and there are also issues due to the rings metal construction (this is why the presence of the absorber material is necessary beneath the NFC tag inlay, it counteracts the effect of the metal ring and basically tricks the inlay antenna into thinking that the metal is further away than it is.)

In essence, the TL;DR is that if your smartphone has a larger than usual 'antenna' loop as part of it's NFC system then the NFC Ring will have a more difficult time being activated by that smartphone's electromagnetic field. This is why finding the sweet spot on your phone is important, it's not just where the loops of the antenna are, it's [i:37zu4v8y]where inside the electromagnetic field[/i:37zu4v8y] your ring activates.

I'm glad you started this thread. I mentioned on the KS comments that starting such a thread would be good, as the coolection came with extra DIY tags and to do something unique with them one would need to know what kinds of materials/constraints would be involved. It also comes into play when setting up any DIY systems for reading tags. Such as placement of an arduino based readers coil.

Also kuddos Lokki for explaining that the tags are using inductive coils rather than an actual antenna.

Hi Logan, I'm glad someone read that mess of a post and came away with something good! ;-)
I was trying to find a way to properly explain the reason behind the 'sweet spot' and that it was an effect springing from both design and implementation and thus unavoidable given the size constraints of a ring as opposed to a credit card. It also helps explain why some of us may need to adjust our expectations a little bit when it comes to the actual usage of the NFC ring, and the reasons behind that.
It's also why the elechouse v3 reader unit is such a damn good unit with the rings, it's design and layout are about as good as it gets for one reason or another. Such a small antenna with very little outside interference from the associated circuitry, the designer has my respect.
The RC522 boards have a similar sized antenna but between the chip used and the placement of the rest of the circuit it just doesn't work well at all.

Thank you for that very elaborate explanation of why the sweet spots are much smaller than some of us had thought. I am still waiting for my normal ring but I have noticed by watching some phones taken apart on the ifixit site that the sweet spots are around edge of the NFC coil. For example the Moto X (which I should receive by mail soon) has the sweet spot which appears to be the bottom left corner on the NFC coil on the phone taken apart on ifixit. Do you think is is just a coincidence or is it because of something like the electromagnetic field being more powerful on the edge of the NFC coil?

Hi mate, thanks for that. I'm extremely glad that it wasn't a wasted half hour of typing and retyping. ;-)
You're exactly right, most sweet spots are on the periphery of the actual antenna loops and it has to do with how the electromagnetic field is being generated and the various things that interfere/interact with it. Basically anything else metal, or that generates an RF/electromagnetic field can interact with the NFC portion of the phone and direct/reflect/distort that electromagnetic field.
The sweet spot is simply the strongest/densest area of RF radiation generated by the phone, so read issues are really down to the individual phone being better or worse at it than the test units were.
I'll be very interested to hear how you go with the ring and where your sweet spot is in relation to the antenna loop.

@Lokki I just got my normal size ring today and as I had predicted the ring can be read all the way around the NFC coil of my Moto X. As long as I have the ring in the same direction (horizontal/vertical) as the coil there isn't really a "sweet spot," even with my case on. I haven't posted anything in the "sweet spot" part of the forum yet, but it is basically identical to this picture of the coil that is posted on ifixit.com - http://d3nevzfk7ii3be.cloudfront.net/igi/suMxHauKHXeQufAD.huge

If it wasn't for your theory I probably would have assumed that the one "sweet spot" from a negative review that was posted on YouTube was the only one I would find without an alpha size ring. Thanks

@lostboardmayhem , wow, that's pretty awesome. From the sound of it the Moto X is one of the best performers I've heard about so far then - and I was impressed with the Xperia line until now.
That's great, thanks for letting us know how you went! Also awesome that you can use the Classic ring so easily with that one!!
Was the negative review on the phone or the NFC Ring?

The negative review was on the ring which he says he only got to work on one device, the Moto X, and in a spot smaller than a "quarter inch." He clearly didn't do something right or try hard enough. I came across the video because John mentioned it on here a while ago, but here is a link:

Wow. Thanks for posting that there @lostboardmayhem .
So yeah, first thoughts on that are for everyone who ever tries to use the NFC Ring as a phone unlock device:

Use the sweet spot sticker if you're having trouble finding the sweet spot. It provides tactile feedback for your ring position.

Mileage really does vary depending how your phone has been treated over it's life with you.

It's really not just about tapping the ring at the phone, it's really not. We're not making click noises on the rear cover to unlock, we're passing the inlay through an invisible RF field which triggers it. This can require a little finesse sometimes but once you get it sorted out you can repeat it again and again with accuracy. Again, sweet spot stickers! They really do help!

Ring orientation will change where it's readable on some phones! Try everything, vertical/horizontal/diagonal!

Moving too fast over the back of the phone isn't going to be a good indicator of anything. I'd have done vertical/horizontal sweeps at least half as quick as he was doing. Patience is key when you are learning something new!

If he held the phone in his hand properly and then moved that finger back and forth through the sweet spot I'm betting it would activate a lot more consistently.

But anyway, that just shows how frustrating it could be if you couldn't quite get things happening properly and were stuck on the idea of tap and read!

@Lokki, thank you for writing on this topic and this has been a much discussed topic in my circles because NFC is still an area widely being researched upon and being improved every day. I use an LG Optimus G Pro and I was wondering recently why it takes so long to connect to a Samsung S4 device. The Samsung S4 has it’s GSM antennae on the sides of the phone whereas the Optimus G Pro have them on the back and they are a little bit wider as compared to those of S4 which in my opinion makes it difficult for the two NFC antennae to communicate. Considering the fact that NFC is going to be widely used in the future, a common protocol needs to be established between all the venders which is subject to changes with time.

Hey, no problem @cilek, thanks for the good words. With the Samsung phones that we see the biggest issues with it's mostly down to antenna size, location and connections.
They opted to go on the back of the battery (predominantly metal object), used all the available space (poorly matched larger antenna) and standard battery style contacts (subject to vibration and movement based wear as well as the effects of tarnish and external elements like moisture).
As far as I'm concerned it's the unholy trinity right there. A smaller more well matched antenna, using proper RF connectors and located a little further away from the battery would have improved things no end.